Based on anti-HIV-1 screening of a repository of 200,000 blood donor sera collected in late 1984, the Transfusion Safety Study identified and enrolled into ongoing followup 133 seropositive donors and 111 HIV-1-infected transfusion recipients. Included among these are 38 "transmission clusters" composed of a donor and from one to six linked, infected recipient(s), and, in four cases, infected recipients' sexual partners. Frozen cells and sera collected at six-month intervals over a six-year period from members of these clusters are available to us. This proposal describes a comprehensive virological study of these well- characterized clusters/specimens. The study approach, methodology, and analyses are designed to assess current models of HIV-1 genetic evolution and pathogenesis, based on the sequence diversity displayed by this lentivirus. Specimens will be accessed such that sub-repositories of cellular DNA, PCR-amplified HIV-1 sequences, and viral isolates will be generated for future investigations of these unique clusters. Our intended studies include analysis of sequence diversity in envelope (env) V3, V4, and V5 hypervariable regions both within each infected individual over time, and between different members of each cluster and different clusters. Sequence diversity profiles from PBMC and serum will be analyzed separately to discern differences in these different blood components at each sampling and over time. The pattern of turnover of specific sequence variants over time (evolution of viral genotypes) will be correlated with clinical and immunological progression. The findings of this study should help elucidate the significance of HIV-1 heterogeneity in disease pathogenesis and the relative contributions of host vs viral strain factors in driving viral diversity (evolution).